damage that prevents a craft takeoff and damage that doesn't prevent craft takeoff.
Remember, you
can launch a craft that has only 1% health left, if it has >0% fuel in it. Therefore "damage that prevents a craft takeoff" cannot logically exist.
Maybe if you'd argue that there exists "damage that prevents a craft from being refuelled", but that also doesn't make sense because, logically, a craft's fuel system works just fine since the craft can take off, fly, and land without issues. Any holes in the fuel tank would only make refuelling easier hehe.
In my opinion, these are not issues, but desired and well designed fundamental geoscape features.
I'd argue that because in xcomapoc these fundamental geoscape features have been changed (so that refuelling rearming and repairing all happen at the same time), it indicates that the original behavior of these features in ufo/tftd was undesirable.
Also, no one is arguing that repairing should be removed altogether. What we can say is that, considering mods do often use the repairRate craft rule, it, again, indicates that unmodded maintenance behavior is undesirable.
Anyway, I agree that if this feature was sufficiently moddable, then a workaround could be produced. What would be a good way to mod this?
You could make a geoscape yankes-script function that gets executed every 30min on every craft, which would contain the repairing/rearming/refuelling logic. Modders could then change this logic and include their own code to modify the craft's repair/refuel/rearm state. I do like Finnik's idea of consuming custom items for repairs, perhaps even custom repair speed depending on the item used, tho that might be a bit hard to realize in a yankes-script function.